Slit coating devices which are also referred to as application heads, are employed in various areas of industry to apply adhesives, sealants or other fluids to extensive areas of various substrates such as films, packaging materials, parts of machines or other work pieces. Such devices are also employed to apply glue to book spines. The device is mounted on a framework or the like, in such a way that the substrate which is to be coated is guided past while the fluid is being dispensed. The fluid flows out of the slit-shaped, elongated discharge openings onto the surface of the substrate. The discharge opening of the elongated slit of the slotted nozzle assembly may either be continuous, in order to apply a continuous strip of fluid, or may be subdivided to dispense and apply a plurality of adjacent strips or narrow strings or beads are dispensed and applied.
To vary the effective length of the essentially slit-shaped discharge opening of the nozzle assembly, the effective length of the transverse distribution channel that is fillable with fluid may be varied. To this end, a piston-like closure element is movably situated in the distribution channel on at least one side of the distribution channel. The closure element is sealed within the distribution channel, so that it delimits the distribution channel laterally in various positions in such a way that the effective length of the distribution channel, and thus the effective length of the slit, and hence of the slit-shaped discharge opening, is variable. The closure element may be connectible for example with a rod, which is movable back and forth manually or motor-driven or with the aid of a threaded spindle apparatus. In this way, it is possible for strips of fluid of differing widths to be dispensed and applied to a substrate. For example, when applying glue to book spines the application width is adjustable to the size of the books. Such a device is known for example from WO 00/67914. In some versions of the slotted nozzle assembly, the closure element is connected to an extension, sometimes referred to as a tab, that projects into the slit in the direction of the discharge opening so that the elongated slit is also limited laterally to the area of the discharge opening. The intent of this is to achieve sharp lateral limits to the strip of fluid being dispensed, and thus also of the strip being applied to the substrate.
When the length of the distribution channel is varied by shifting a closure element, the volume of the distribution channel changes. In those cases in which the closure element is connected to a which projects into the slit to the area of the discharge opening, the volumes of the distribution channel and of the slit. This change of volume can result in disadvantages when the length is varied. During a reduction of the application width by shortening the distribution channel, adhesive may be pressed out of the slit-shaped discharge opening, and this adhesive may discharge onto the substrate or the surroundings in an unwanted manner. When the volume of the distribution channel, and possibly of the slit, is increased, air may flow from the environment into the slit and the distribution channel while the volume of the distribution channel is increasing. When the application operation is resumed, the drawn in air may delay adhesive flow from the discharge opening, resulting in adequate coating of the substrate. This can result, for example, in a book spine being incompletely glued, so that it subsequently cannot be processed further.
The object of the present invention is therefore to provide a device for dispensing fluid onto a substrate, where the change in the effective length of the slit does not result in the disadvantages described above. In particular, a device for dispensing fluid is to be provided which immediately achieves optimal coating results, even after a change to the slit length and thus to the application width.